1,6-Dioxo-2-azaspiro[3.4]oct-2-enes and Related Spirocycles: Heterocycles from [3 + 2] Nitrile Oxide Cycloadditions with 2-Methyleneoxetanes, -Thietanes, and -Azetidines.

Isoxazolines and 4-membered heterocycles are significant structural motifs in numerous synthetic intermediates and natural products. [3 + 2] Cycloadditions between enol ethers and nitrile oxides have been well studied; however, nitrile oxide cycloadditions with 4-membered heterocycles to give spiroisoxazolines are unreported. Here, we showcase the regio- and diastereoselective [3 + 2] nitrile oxide cycloadditions of 2-methyleneoxetanes, -azetidines, and -thietanes to give an array of 1,6-dioxo-2-azaspiro[3.4]oct-2-enes and related spirocycles. 2D NMR experiments suggested that most of the observed diastereoselectivities were dictated by steric interactions; however, dipolarophiles with H bonding donors reversed the stereochemical outcome. X-ray crystallography confirmed the structural assignments.